Passive, cavity-backed, sinuous and log-periodic antennas typically have a ˜10:1 bandwidth and require larger sizes to go to lower frequencies. These incorporate baluns and impedance matching networks to arrive to 50 Ohm terminal impedance for connection to coax cables. An existing product that is on the market today is sold, for example, by L3 Randtron of Menlo Park, Calif. The impedance match bandwidth may be extended to lower frequencies by adding loss, which rapidly degrades sensitivity.
Feedback LNAs are known in the art but are typically matched to 50 Ohms and are not integrated into these types of antennas. Rather, they are separated by a transmission line that requires a 50 Ohm match from the antenna to avoid standing waves and their associated ripple in the frequency response.
The presently disclosed technology relates to a wideband receive antenna that operates both in a “traditional band” of operation, where the size is >=½ wavelength (λ) at the minimum frequency in that band, and also in a low-frequency “extension band,” where the antenna size is <λ/2. A boundary frequency fc is defined as that frequency which defines a boundary between the “traditional band” of operation and the “extension band” of operation. The presently disclosed antenna is a wideband antenna (preferably a cavity-backed sinuous antenna, but it also encompasses log-periodic and other types of antennas having N arms, N feed terminals, and an array of N buffer amplifiers integrated directly into or at the feedpoint of the antenna. N is greater than 1 and typical values of N may be 2 or 4, for example. Other values of N (>1) are also possible. “Directly into the feed or feedpoint” means that any transmission line used to connect the radiating arms of the antenna to a transistor input element of the buffer amplifiers (or example, the gate of a FET amplifier) is much shorter than a wavelength at any frequency in the extension band (preferably less than 0.1 wavelength at any frequency in the extension band) and also preferably less than ¼ wavelength at any frequency in the traditional band. The buffer amplifiers are preferably configured to possess high gain and a low noise figure when noise-matched to an antenna impedance Za (typically different than 50 Ohms). Za is chosen as the input impedance of the antenna arms in the traditional band. The buffer amplifiers are tied to a common ground node that is floating relative to the antenna arms. These buffers preferably comprise GaN FET transistors and are preferably implemented on a single semiconductor die or module. The buffer outputs are preferably impedance matched to an interface impedance (typically 50, 75 or 100 Ohms) and may be further coupled to either a combining network or to N receivers. As will be disclosed, the receive antenna may also be used as a transmit antenna, even though the present disclosure is primarily directed to its receive functionality.
A purpose of the presently disclosed technology is to make receive antennas have a wider bandwidth than is possible with state of the art antennas without increasing their size. Traditional wideband cavity-backed antennas operate over a 10:1 bandwidth and are between 0.5λ and 1λ in size at their operational minimum frequency. For example, an antenna operating from 2-18 GHz is ˜2.4 inches or more in size. Increasing the bandwidth requirement to 0.5-18 GHz would mean increasing the size to ˜10 inches (and would introduce additional design challenges to maintain the impedance match), or would force the user to accept severely degraded receive sensitivity (i.e. minimum detectable signal) over the 0.5-2 GHz extension band. This invention may allow operation over 0.5-18 GHz with a 2.4 inch size without severely degrading the sensitivity.
Prior art devices do not exist, to our knowledge, which anticipate this invention. Prior art antennas have been combined with Low Noise Amplifiers (LNAs), but that prior art does not achieve this bandwidth extension found with the present invention.
The presently disclosed technology addresses a long-felt need for wide bandwidth in a small antenna. In addition, this invention runs counter to textbook teaching on antenna and low-noise amplifier design.